Chapter 10
Server Load Balancing (SLB)
See the following sections to configure and use these features:
■ 10-1: SLB: Covers the configuration steps needed to provide load balancing of traffic
to one or more server farms
■ 10-2: SLB Firewall Load Balancing: Discusses the configuration steps necessary to
load balance traffic to one or more firewall farms
■ 10-3: SLB Probes: Explains the configuration steps needed to define probes that test
server and firewall farm functionality
10-1: SLB
■ SLB provides a virtual server IP address to which clients can connect, representing a
group of real physical servers in a server farm. Figure 10-1 shows the basic SLB con-
cept. A client accesses a logical “virtual” server (IP address v.v.v.v), which exists only
within the Catalyst 6500 SLB configuration. A group of physical “real” servers (IP ad-
dresses x.x.x.x, y.y.y.y, and z.z.z.z) is configured as a server farm. Traffic flows be-
tween clients and the virtual server are load balanced across the set of real servers,
transparent to the clients.
■ As clients open new connections to the virtual server, SLB decides which real server
to use based on a load-balancing algorithm.
■ Server load balancing is performed by one of these methods:
■ Weighted round-robin: Each real server is assigned a weight that gives it the
capability to handle connections, relative to the other servers. For a weight n, a
server is assigned n new connections before SLB moves on to the next server.
■ Weighted least connections: SLB assigns new connections to the real server
with the least number of active connections. Each real server is assigned a
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